Compounds having the following structure wherein R.sup.1, R.sup.2,
R.sup.3, R.sup.4, R.sup.5, R.sup.6, R.sup.7 m and n are as defined in the
specification, in vivo-hydrolysable precursors thereof,
pharmaceutically-acceptable salts thereof, the use in therapy and
pharmaceutical compositions and methods of treatment using the same.